Organic thin-film solar cells are formed by a coating technique with a solution of an organic compound, which is a photoelectric conversion material. The cells have various advantages: for example, 1) device production cost is low; 2) area expansion is easy; 3) the cells are more flexible than inorganic materials, such as silicon, thus enabling a wider range of applications; and 4) resource depletion is less likely. As such, organic thin-film solar cells have been developed, and the use of the bulk heterojunction structure has particularly led to a significant increase in photoelectric conversion efficiency, thus attracting widespread attention.
For p-type semiconductor of the photoelectric conversion basic materials used for organic thin-film solar cells, poly-3-hexylthiophene (P3HT) is particularly known as an organic p-type semiconductor material exhibiting excellent performance. With an aim to obtain advanced materials, recent developments have provided compounds (donor-acceptor type π-conjugated polymers) that can absorb broad wavelengths of solar light or that have tuned energy levels, leading to significant improvements in the performance of materials. Examples of such compounds include poly-p-phenylenevinylene and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7).
For n-type semiconductors as well, fullerene derivatives have been intensively studied, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been reported as a material having excellent photoelectric conversion performance (see the below-listed Patent Documents 1, 2, etc.). Nonetheless, there have been few reports that demonstrate stable and excellent photoelectric conversion efficiency of fullerene derivatives except for PCBM.
Although fullerene derivatives for organic solar cells other than PCBM have been reported, the reports concern a comparison using special devices from which a power collection material of the positive electrode (ITO electrode) is removed (Non-patent Document 1), or fullerene derivatives only showing performance almost equivalent to that of PCBM (Non-patent Document 2). Moreover, although the disubstituted derivatives reported by Y. Li et al. (Non-patent Document 3), when used with P3TH, achieved higher conversion efficiency than PCBM as reported by E. T. Hoke et al., the disubstituted derivatives exhibited only low conversion efficiency when used with a donor-acceptor type π-conjugated polymer (Non-patent Document 4).
Thus, except for PCBM, advanced n-type materials capable of achieving high conversion efficiency, independently of p-type materials, have been unknown.
Several methods for synthesizing fullerene derivatives have been proposed. Methods known to be excellent from the standpoint of yield and purity include a method for synthesizing, using a diazo compound, a fullerene derivative having a 3-membered ring moiety and a method for synthesizing a fullerene derivative having a 5-membered ring moiety to which an azomethine ylide generated from a glycine derivative and an aldehyde is added.
The aforementioned PCBM is a fullerene derivative having a 3-membered ring moiety, and PCBM can be obtained by preparing a mixture of three types of products each having a fullerene backbone to which a carbene intermediate is added, and subjecting the mixture to a conversion reaction by light irradiation or heat treatment. However, the derivative having a 3-membered ring moiety obtained by this production method is restricted in terms of the introduction site of substituent and the number of substituents; thus, the development of novel n-type semiconductors has significant limitations.
Fullerene derivatives having a 5-membered ring moiety, on the other hand, are considered to be excellent because of their diverse structures. However, there have been few reports on the fullerene derivatives having excellent performance as an n-type semiconductor material for organic thin-film solar cells. One of a few examples is the fullerene derivative disclosed in the below-listed Patent Document 3.